The present invention relates to methods of detecting vehicle bearings and of processing data in a vehicle navigation system, and more particularly to methods of detecting the bearing of a vehicle using a terrestrial magnetism sensor and an angular velocity sensor and of processing the data obtained.
Research and development activities are being directed to providing a vehicle navigation system for guiding a vehicle to an intended destination by having not only map information but also the present vehicle location displayed on a display unit, the map information being prestored in and read out of a memory.
The navigation system must be equipped with a bearing sensor for detecting the bearing of the vehicle. For such a bearing sensor, there may be employed a terrestrial magnetism sensor for detecting the bearing of the vehicle according to terrestrial magnetism (the magnetic field of earth). Also, there may be used an angular velocity sensor for detecting the bearing of the vehicle by detecting the vehicle's angular velocity. However, the terrestrial magnetism sensor is easily affected by external disturbances, and its output tends to include a gross error when the vehicle passes a railroad crossing, an iron or steel bridge, a large vehicle (e.g., a truck or bus), or the like.
Moreover, because the bearing obtained from the output data of the angular velocity sensor is not an absolute bearing, drift in the calculated bearing may occur.
To obtain the bearing of the vehicle from the output data of the angular velocity sensor, the data supplied by the angular velocity sensor when no angular velocity exists (i.e., when the vehicle is not turning) is assumed, as the central value G.sub.dc of the sensor output. The displacement is then obtained according to the central value G.sub.dc and it is integrated with respect to time to obtain the bearing. However, the central value G.sub.dc may fluctuate due to temperature changes. If the central value G.sub.dc fluctuates when the bearing is computed, the error caused thereby becomes a drift of the bearing. Also, the characteristics of the circuit (e.g., operational amplifier) used for detecting the output from the angular velocity sensor may cause the angle of rotation obtained from the output data of the angular velocity sensor not to be 90 degrees even though the vehicle has actually turned in a 90-degree arc. For this reason, there must be applied a coefficient of error correction. If the angle of rotation of the vehicle thus obtained is still inaccurate when the coefficient of correction is applied, an additional error will become unavoidable.